Sound insulation component for a rigid structural member of an automobile

ABSTRACT

A sound-insulation component for a rigid structural member, of the type including a mass-spring system ( 12 ) includes at least one layer of a heavy mass ( 16 ) and one layer defining a spring ( 14 ), and a sound absorption system ( 18 ) with two layers having different air-flow resistances. The structural member is a metal sheet, the mass-spring system ( 12 ) is provided on one side of the rigid structural member ( 10 ), and the sound absorption system ( 18 ) is provided on the other side of the rigid structural member ( 10 ). The first layer ( 22 ) among the two layers of the sound absorption system ( 18 ) consists of a non-woven fabric having an adjustable air-flow resistance. Application in the sound insulation of automobiles.

The present invention relates to a sound-proofing component for a rigidstructural member, in particular of a vehicle.

In a vehicle, it is desirable to sound-proof the passenger compartment.To this end, there is used an “insulation” which prevents medium- andhigh-frequency sound waves from entering the sound-proofed spacesubstantially by reflecting the waves towards the sources of noise oroutside the sound-proofed space, and “acoustic absorption” whichdissipates the energy of the sound waves (at medium and highfrequencies) in an absorbent material.

It is relatively simple to obtain good sound-proofing at highfrequencies using simple means but, in the range of medium frequencies(in particular between 400 and 1000 Hz), the problem of sound-proofingis all the more acute because the human ear is very sensitive in thisfrequency range.

For this sound-proofing, a mass/spring system is already used andsubstantially provides insulation, in particular in the region of thebulkhead which separates the passenger space from the engine compartmentof a vehicle. A layer of a material which has a spring-like effect, suchas a foam which may optionally have viscoelastic properties, carries alayer of a material of the “heavy mass” type, that is to say, the layerconstituting the mass of the mass/spring system. Such a mass/springsystem is known for the good insulation which it provides. Document GB-2163 388 describes such a mass/spring system whose heavy mass portioncomprises two layers.

The problem presented by this system is that, owing to its low level ofabsorption, the layer which forms the heavy mass must have a significantmass surface density in the order of from 3 to 7 kg/m². Furthermore,such a system is not very effective in the medium frequency range.

Since it is desirable to make vehicles lighter in order to reduceconsumption, pollution, etcetera, document WO 98/18 657 has proposed asystem which has two layers with differing levels of resistance to airpassage. In this system, a porous decoupling layer is in contact withthe structural support member, such as a metal sheet, preferably withpartial interposition of air, and it is covered by another porous layer22. In one example, one layer is a compressed phenolic felt and theother a flexible non-compressed felt. The action of this system is aresult of its dual-permeability, that is to say, the difference inresistance to air passage between the porous layers. The advantage ofthis system is that it is light, but the disadvantage thereof is that itprovides practically no insulation.

Document WO 03/069 596 describes a complex sound-proofing system whichcomprises two groups of layers: one group of layers comprises a layer ofthe heavy mass type which is associated with a porous layer which formsa spring. This group constitutes a spring/mass system of theconventional type but the mass of the heavy layer and the thickness ofthe layer which forms a spring are reduced, for example, by from a thirdto a half, compared with the conventional system. The other group oflayers comprises a porous layer of the acoustic spring type and anexternal layer which has a high level of resistance to air passage. Thisexternal layer may be a felt which has a level of resistance to airpassage which is from 3 to 20 times greater than that of the otherporous layer.

This document WO 03/069 596 thus describes an improved system whichcombines the mass/spring system with the system having a sound-proofingcomplex which has layers which have differing levels of resistance toair passage and provides excellent sound-proofing results with a reducedweight.

An improvement of the sound-proofing assembly described in this documentWO 03/069 596 has also been envisaged, that is to say, which comprisessuperimposed layers and which comprises a first group of layers whichhave a good level of resistance to air passage and a second group oflayers which has a mass/spring function; according to this improvement,the first group of layers is produced so that it comprises a layer of afoam with open cells with a high level of porosity, a high level ofcomplexity and good resistance to the passage of air, this layer having,owing to its high level of complexity, excellent sound absorptionproperties at medium and high frequencies. The complexity, whichcorresponds to the inclination of the line illustrating the variation ofthe square of the refractive index for the acoustic wavelength used as afunction of the inverse of the square root of the frequency is connectedto the shape of the circulation paths in a porous material. The foamswith open cells which are commonly used have a complexity of between 1and 1.35 and a high level of complexity, preferably in the order of from1.5 to 2.5.

In this manner, according to this improvement, the “sound-proofing”complex of the document WO 03/069 596 is replaced by a single layer of amaterial with a high level of complexity.

It has now been realised according to the invention that, when thesound-proofing system described in the above-mentioned document WO03/069 596 or the above-mentioned improvement thereof was separated intotwo portions which are placed at one side and the other of theassociated rigid structural member, it was possible to obtain, withidentical constituent elements, either much better sound-proofing, or,with practically identical sound-proofing, a reduction in the weight ofthe component of at least 25% compared with the conventional mass/springsystem.

Document WO 99/58371 describes a sound-proofing component which includesa structural member and which is constituted by a rigid protective platewhich is mounted at the outer side of the structural member and so as tobe remote therefrom, and an optional acoustic insulator which is mountedbetween the structural member and the protective plate.

Document WO 01/92086 describes an embodiment of a floor sound-proofingcomponent whose main element is a “thick” structural member, that is tosay, an element which is constituted by a core in the form of ahoneycomb surrounded by two rigid layers. This core in the form of ahoneycomb forms a thick layer of air.

More precisely, according to the invention, the portion which forms thesound-proofing complex which has layers whose levels of resistance toair passage are different is arranged at the side of the structuralelement which is directed towards the source of noise, and themass/spring system is arranged at the side to be sound-proofed, forexample, at the side of the passenger compartment of a motor vehicle.

On the bulkhead which separates the engine compartment from thepassenger space, a single layer of foam or felt is sometimes arrangedand is occasionally provided with a decorative element. This layer,which is incorporated for reasons of protection and aesthetics, has nosound-proofing function. This is particularly a result of the fact thatthis layer covers only a portion of the surface of the bulkhead, so thatthe portions of bulkhead which are not covered form a short-circuit orpreferred acoustic path which cancels out the sound-proofing effectwhich such a layer could have.

The invention eliminates this effect involving an acoustic short-circuitsince the opposing surfaces of the structural member which are coveredby the mass/spring system on the one hand and the sound-proofing complexor acoustic absorption system on the other hand are practically oppositeeach other and of the same size.

More precisely, the invention relates to a sound-proofing component fora rigid structural member, of the type comprising a mass/spring systemwhich comprises at least one layer of heavy mass and a layer which formsa spring, and an acoustic absorption system having two layers which havediffering levels of resistance to air passage; according to theinvention, the structural member is a metal sheet, the mass/springsystem is arranged at one side of the rigid structural member, theacoustic absorption system is arranged at the other side of the rigidstructural member, and the opposing surfaces of the structural memberwhich are covered by the mass/spring system on the one hand and theacoustic absorption system on the other hand are opposite each other andof the same size.

Preferably, a first of the two layers of the acoustic absorption systemhas an adjustable level of resistance to air passage. Preferably, thisfirst layer is formed by a non-woven material. The non-woven material isadvantageously based on unsaturated polyester combined with cellulose or“Nylon”.

Preferably, the second of the two layers is porous and flexible.Preferably, the second of the two layers is formed by a materialselected from a felt or a foam.

In one embodiment, the acoustic absorption system comprises a layer offoam which has a high level of complexity.

Preferably, the mass/spring system comprises a layer which forms aspring and which is adjacent to the rigid structural member. Forexample, the layer which forms a spring is a foam.

In a particularly efficient embodiment, the component further comprises,at the side of the mass/spring system opposite the rigid structuralmember, a sound-proofing complex which is formed either by two layerswhich have different levels of resistance to air passage, or a layer offoam with a high level of complexity.

In embodiments, the rigid structural member is a metal sheet whichseparates the passenger compartment of a vehicle from a space selectedfrom an engine compartment, a wheel housing and an exhaust device.

Other features and advantages of the invention will be better understoodfrom a reading of the following description with reference to theappended drawings, in which:

FIG. 1 is a conventional system of the type described in document WO03/069 596;

FIG. 2 is a sound-proofing system according to the invention;

FIG. 3 comprises an improved system in accordance with document WO03/069 596 compared with the system illustrated in FIG. 2; and

FIG. 4 is an improved system in accordance with the improvement ofdocument WO 03/069 596 compared with the system illustrated in FIG. 2.

FIG. 1 is a schematic section of the structure of a sound-proofingcomponent according to document WO 03/069 596 associated with a rigidstructural member 10, for example, a metal sheet.

The component comprises a mass/spring system 12 which comprises a layer14 which forms a spring, for example, a felt or a foam, and a layer 16of heavy mass, for example, based on an elastomer which is charged witha mineral material.

The component also comprises a system or complex 18 for sound-proofingby means of acoustic absorption which comprises a layer 20, for example,of felt or foam, which has a pre-determined resistance to air passage,and a porous layer 22 which has a high level of resistance to airpassage, this resistance preferably being adjustable. This outer layer22 is advantageously formed by a non-woven fabric. Although it is notillustrated, a carpet may be arranged on the layer 22.

The sound-proofing component illustrated in FIG. 1 providessound-proofing characteristics which are equivalent to those of acomponent of the mass/spring system, but with a weight reduction of upto 50% in some cases. It is therefore very advantageous to use such acomponent for the low weight thereof.

It has now been realised according to the invention that, when thecomponent described in document WO 03/069 596 was used, if themass/spring system 12 was placed at one side of the structural member 10and the complex 18 for sound-proofing by means of absorption at theother side of this structural member 10, even better sound-proofingproperties were obtained.

FIG. 2 illustrates such a system. The same references as in FIG. 1 havebeen used to refer to elements which are similar.

When the system illustrated in FIG. 2 is produced, that is to say, inwhich the mass/spring system 12 is positioned towards the passengercompartment of a vehicle and the sound-proofing complex 18 is positionedtowards the engine compartment, the acoustic insulation increases from10 to 35 dB between 1000 and 8000 Hz compared with the system describedin the above-mentioned document WO 98/18 657.

Since it is desirable to reduce the weight of the components of motorvehicles, this property, for similar acoustic performance levels,corresponds to a reduction in weight of the sound-proofing component inthe order of 25% compared with the conventional mass/spring system.

FIG. 3 illustrates a variant of the component of FIG. 2 providing aneven greater level of sound-proofing. In this component, a complex 18′for sound-proofing by means of absorption which is similar to thecomplex 18 but which may be thinner, is arranged inside so as toco-operate with the mass/spring system 12. Such a system allows areduction in weight of up to 35% compared with the conventionalmass/spring system.

The natures and the characteristics of the different layers used may beidentical to those which are described in the above-mentioned documentWO 03/069 596.

FIG. 4 illustrates another variant of the component of FIG. 2 which alsoprovides even greater sound-proofing and which corresponds to theimprovement already described for the component of the above-mentioneddocument WO 03/069 596. The references which are identical to those ofFIG. 3 refer to elements which are similar.

In this component, the complex 18′ for sound-proofing by means ofabsorption is replaced by a single layer 18″ with a high level ofcomplexity, for example, a layer which is formed by a polyurethane foamhaving a complexity close to 2, and which has a mass surface density of0.6 kg/m² and a thickness of 10 mm.

According to another improvement compared with the documents which havebeen described above, it is particularly advantageous to use, for thelayers 22, 22′, light thermo-formable non-woven materials, for example,based on unsaturated polyester associated with cellulose or “Nylon”(“Ahlstrom” or “3M”) which may provide levels of resistance to airpassage of from 300 to 2500 N.s/m³ for a mass surface density of fromonly 20 to 200 g/m². Such non-woven fabrics readily allow an adjustableresistance to be obtained with respect to air passage.

It should be noted that the component illustrated in FIG. 2 is moreeffective at levels approaching medium frequencies, whilst the componentillustrated in FIG. 3 is more effective at levels approaching highfrequencies.

The layer most remote from the structural member, towards the interiorof the vehicle, may constitute a decorative layer or carry such a layer.

Owing to a layer of the acoustic absorption system which has anadjustable level of resistance to air passage and which isadvantageously formed by a non-woven material, associated with aflexible porous layer, providing an effect of differing resistance toair passage, the acoustic properties may be optimised, in particular inaccordance with the acoustic characteristics of each of the two sides ofthe structural member: nature of the source of noise (engine noise,travel noise, exhaust noise, etcetera..), properties of the receivingcavity (passenger compartment, etcetera . . . ), etc.

For constituent elements which are identical to those of asound-proofing component according to document WO 98/18 657, asound-proofing component according to the invention provides an increaseof from 10 to 35 dB for the insulation.

Compared with the conventional mass/spring system and with practicallyidentical acoustic performance levels, the weight gain may reach 25% inthe case of FIG. 2 and 35% in the case of FIG. 3, in accordance with theshape of the emission and receiving cavities.

There are numerous portions of the vehicle to which such components arewell suited in addition to the partition which separates the enginecompartment from the passenger compartment. It is possible to mention inparticular the wheel housings, floors and the components close to theexhaust.

In this manner, according to the invention, the sound-proofing complex,which functions substantially by means of acoustic absorption, ispositioned at the side of the source of noise; it thus ensures that thesound-proofing component is adapted to the emission spectrum of thesource of noise (for example, the engine or the movement of the tyres),whilst the other portion which is constituted by a simple or improvedmass/spring system in accordance with the above-mentioned document WO03/069 596 or the improvement thereof (FIGS. 3 and 4) substantiallyprovides the insulation of the passenger compartment. Consequently, thismass/spring system which is simple or improved, may be practically thesame for the various sources of noise, that is to say, regardless of thelocation thereof in the passenger space (front bulkhead, floor or wheelhousing).

1. Sound-proofing component for a rigid structural member, of the typecomprising a mass/spring system (12) which comprises at least one layer(16) of heavy mass and a layer (14) which forms a spring, and a system(18) for acoustic absorption having two layers which have differinglevels of resistance to air passage, characterised in that thestructural member (10) is a metal sheet, the mass/spring system (12) isarranged at one side of the rigid structural member (10), the acousticabsorption system (18) is arranged at the other side of the rigidstructural member (10), and the opposing surfaces of the structuralmember which are covered by the mass/spring system (12) on the one handand the acoustic absorption system (18) on the other hand are oppositeeach other and of the same size.
 2. Sound-proofing component accordingto claim 1, characterised in that a first (22) of the two layers of theacoustic absorption system (18) has an adjustable level of resistance toair passage.
 3. Sound-proofing component according to claim 2,characterised in that the first (22) of the two layers of the acousticabsorption system (18) is formed by a non-woven material. 4.Sound-proofing component according to claim 3, characterised in that thenon-woven material is based on an unsaturated polyester combined withcellulose or “Nylon”.
 5. Sound-proofing component according to claim 1,characterised in that the second (20) of the two layers of the acousticabsorption system (18) is porous and flexible.
 6. Sound-proofingcomponent according to claim 5, characterised in that the second (20) ofthe two layers of the acoustic absorption system (18) is formed by amaterial selected from a felt and a foam.
 7. Sound-proofing componentaccording to claim 1, characterised in that the acoustic absorptionsystem (18) comprises a layer of foam which has a high level ofcomplexity.
 8. Sound-proofing component according to claim 1,characterised in that the mass/spring system (12) comprises a layerwhich forms a spring (14) and which is adjacent to the rigid structuralmember (10).
 9. Sound-proofing component according to claim 1,characterised in that it further comprises, at the side of thespring/mass system (12) opposite the rigid structural member (10), asound-proofing complex (18′) which is selected from a complex which hastwo layers (20′, 22′) which have differing levels of resistance to airpassage, and a complex formed by a layer (18″) of foam which has a highlevel of complexity.
 10. Sound-proofing component according to claim 1,characterised in that the rigid structural element (10) is a metal sheetwhich separates the passenger compartment of a vehicle from a spaceselected from an engine compartment, a wheel housing and an exhaustdevice.
 11. Sound-proofing component according to claim 2, characterisedin that the second (20) of the two layers of the acoustic absorptionsystem (18) is porous and flexible.
 12. Sound-proofing componentaccording to claim 2, characterised in that the acoustic absorptionsystem (18) comprises a layer of foam which has a high level ofcomplexity.
 13. Sound-proofing component according to claim 2,characterised in that the mass/spring system (12) comprises a layerwhich forms a spring (14) and which is adjacent to the rigid structuralmember (10).
 14. Sound-proofing component according to claim 2,characterised in that it further comprises, at the side of thespring/mass system (12) opposite the rigid structural member (10), asound-proofing complex (18′) which is selected from a complex which hastwo layers (20′, 22′) which have differing levels of resistance to airpassage, and a complex formed by a layer (18″) of foam which has a highlevel of complexity.
 15. Sound-proofing component according to claim 2,characterised in that the rigid structural element (10) is a metal sheetwhich separates the passenger compartment of a vehicle from a spaceselected from an engine compartment, a wheel housing and an exhaustdevice.
 16. Sound-proofing component according to claim 3, characterisedin that the second (20) of the two layers of the acoustic absorptionsystem (18) is porous and flexible.
 17. Sound-proofing componentaccording to claim 3, characterised in that the acoustic absorptionsystem (18) comprises a layer of foam which has a high level ofcomplexity.
 18. Sound-proofing component according to claim 3,characterised in that the mass/spring system (12) comprises a layerwhich forms a spring (14) and which is adjacent to the rigid structuralmember (10).
 19. Sound-proofing component according to claim 3,characterised in that it further comprises, at the side of thespring/mass system (12) opposite the rigid structural member (10), asound-proofing complex (18′) which is selected from a complex which hastwo layers (20′, 22′) which have differing levels of resistance to airpassage, and a complex formed by a layer (18″) of foam which has a highlevel of complexity.
 20. Sound-proofing component according to claim 3,characterised in that the rigid structural element (10) is a metal sheetwhich separates the passenger compartment of a vehicle from a spaceselected from an engine compartment, a wheel housing and an exhaustdevice.